Reducing CO2 footprint by route optimization of supply of materials for assembly lines

The project “Reducing CO₂ footprint by route optimization of supply of materials for assembly lines” was motivated by a situation on the shop floor at the Volkswagen (VW) in Navarra assembly plant and is based on two mixed-model assembly lines at VW-Navarra and the aim of the project is to optimize the feeding process in Volkswagen car factory with the aim of reducing the energy consumption and consequently reducing the environmental pollutions.

In VW-Navarra, parts are categorized as high demand and low demand. There is a main warehouse and one supermarket, which is close to each assembly line. According to the VW-Navarra part feeding policy, three strategies are available to supply the assembly lines. The first is transporting the parts from the warehouse directly to the assembly line in big containers; the second is transporting the parts in small bins directly from the warehouse to the assembly line; the third is transporting the parts to the supermarket (in the case where excessive operation is required, e.g., repackaging the big containers into small bins to have frequent small-lot deliveries by means of two trains) and supply the assembly stations through the supermarket. Since a big portion of the parts used in the assembly operation is supplied from the supermarket in small bins, the focus of this study is on the third supplying process.

Small bins are fed to the assembly line by means of tow trains, and wagons are loaded with bins in the supermarket. A tour begins at the supermarket, goes to the assembly plant according to a predefined schedule in order to deliver the full bins to the corresponding assembly stations, and then collects the empty bins. The empty tow train returns to the supermarket to be reloaded for the next tour. The tours frequently take place during each working shift with the aim of reducing the inventory level in the assembly line.

The production plan is usually known a few days before production, and thus the exact consumption rate of each part between two consecutive visits at each station can be calculated for the planning horizon. Consequently, bins can be loaded on tow trains in the supermarket with a fixed and predetermined consumption rate for each part, which consequently enables reliable scheduling in the defined planning horizon. The planning horizon is daily and the number of tours each day is 15 (5 tours per shift). The storage capacity restriction is known for each station and all the parts are delivered in small, standardized bins with a specific capacity for each part.

The capacity limit for each wagon of a tow train is almost 50 bins, and no more than two wagons are allowed per tow train. Furthermore, each assembly line is supported by a single supermarket, which makes it easier and more practical by avoiding any interruption since all the requests and deliveries are managed at a single place near the assembly line.

In addition, the assembly lines are two-sided where both sides of the line (left and right) are used in parallel.

In the frist phase of the project we have studied and modeled the feeding system. In the next step of the project our aim is to include the environmental issues in the model in order to analyze the impact of production decisions in the CO₂ footprint.